| Summary: | Three hydrochemical types of CO<sub>2</sub>-rich water (i.e., Na-HCO<sub>3</sub>, Ca-Na-HCO<sub>3</sub> and Ca-HCO<sub>3</sub>) occur together in the silicate bedrock (granite and gneiss) of Gangwon Province in South Korea. As a natural analogue of geological carbon storage (GCS), this can provide implications for the environmental impacts of the leakage of CO<sub>2</sub> from deep GCS sites. By using hydrochemical and isotopic datasets that were collected for previous and current studies, this study aimed to carefully scrutinize the hydrochemical differences in the three water types with an emphasis on providing a better understanding of the impacts of long-term CO<sub>2</sub> leakage on groundwater quality (especially the enrichments of minor and trace metals). As a result, the Na-HCO<sub>3</sub> type CO<sub>2</sub>-rich water contained higher Li, Rb and Cs than the Ca-HCO<sub>3</sub> type, whereas Fe, Mn and Sr were higher in the Ca-HCO<sub>3</sub> type than in the Na-HCO<sub>3</sub> type despite the similar geological setting, which indicate that the hydrochemical differences were caused during different geochemical evolutionary processes. The δ<sup>18</sup>O and δD values and tritium concentrations indicated that the Na-HCO<sub>3</sub> type was circulated through a deep and long pathway for a relatively long residence time in the subsurface, while the Ca-HCO<sub>3</sub> type was strongly influenced by mixing with recently recharged water. These results were supported by the results of principal component analysis (PCA), whose second component showed that the Na-HCO<sub>3</sub> type had a significant relation with alkali metals such as Li, Rb and Cs as well as Na and K and also had a strong relationship with Al, F and U, indicating an extensive water-rock interaction, while the Ca-HCO<sub>3</sub> type was highly correlated with Ca, Mg, Sr, Fe and Mn, indicating mixing and reverse cation exchange during its ascent with hydrogeochemical evolution. In particular, the concentrations of Fe, Mn, U and Al in the CO<sub>2</sub>-rich water, the result of long-term water-rock interaction and cation exchange that was enhanced by CO<sub>2</sub> leakage into silicate bedrock, exceeded drinking water standards. The study results show that the leakage of CO<sub>2</sub> gas and CO<sub>2</sub>-rich fluid into aquifers and the subsequent hydrogeochemical processes can degrade groundwater quality by mobilizing trace elements in rocks and consequently may pose a health risk.
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